Before air can be introduced into a cryogenic air separation process in which oxygen and nitrogen are separated from one another, it is necessary to remove carbon dioxide present in the air at low levels, e.g. 400 ppm. If this is not done, the carbon dioxide will solidify in the air separation plant. Two methods generally used for such carbon dioxide removal are pressure swing adsorption (PSA) and temperature swing adsorption (TSA).
In each of these techniques, a bed of adsorbent is exposed to a flow of feed air for a period of time to adsorb CO2 from the air. Thereafter, the flow of feed air is shut off from the adsorbent bed and the adsorbent is exposed to a flow of purge gas which strips the adsorbed CO2 from the adsorbent and regenerates the adsorbent for further use. In TSA, the CO2 is driven off from the adsorbent by heating the adsorbent in the regeneration phase. In PSA, the pressure of the purge gas is lower than that of the feed gas and the change in pressure is used to remove the CO2 from the adsorbent.
Other components can be removed from the feed air by these processes, including hydrocarbons and water. These adsorption techniques can also be applied to feed gases other than air or to air to be purified for purposes other than use in an air separation plant. For example, other applications where trace or dilute CO2 needs to be removed prior to a cryogenic separation include the cryogenic production of CO from synthesis gas, the production of liquefied natural gas, and production of dry CO2-free air for a variety of applications.
The use of PSA for removing CO2 from air prior to cryogenic air separation is described in numerous publications, e.g. U.S. Pat. No. 4,249,915 and U.S. Pat. No. 4,477,264. Initially, the practice was to use a dual bed of alumina for water removal followed by a zeolite such as 13× for CO2 removal. More recently, all alumina PSA systems have been proposed, as described in U.S. Pat. No. 5,232,474. The advantages of an all alumina system include lower adsorbent cost, vessel design which does not need screens to separate the two different adsorbents, and better thermal stability in the adsorption vessel during blowdown and repressurization. It would be desirable however, to develop adsorbents having an improved CO2 capacity so as to allow smaller bed sizes with lower capital costs and less void gas being lost during depressurization i.e. higher air recoveries.
Alumina is also used as an adsorbent in TSA and for this purpose it has been proposed to treat the alumina to form alkali metal oxides thereon to increase the adsorptive capacity of the alumina. By way of example, U.S. Pat. No. 4,493,715 teaches a method for removing CO2 from olefin streams by contacting the feed gas with a regenerable, calcined adsorbent consisting essentially from 1 to 6 wt % of an alkali metal oxide selected from the group consisting of sodium, potassium, and lithium on alumina. The adsorbent was prepared by contacting alumina with an alkali metal compound which is convertible to the metal oxide on calcination.
U.S. Pat. No. 4,433,981 describes a process for removing CO2 from a gaseous stream which comprises contacting the gas steam at a temperature up to about 300° C. with an adsorbent prepared by impregnation of a porous alumina with a sodium or potassium oxide. The corresponding oxide can be prepared by impregnation with a decomposable salt and calcining at a temperature of 350° C. to 850° C. Salts mentioned include alkali metal bicarbonates.
U.S. Pat. No. 3,557,025 teaches a method to produce alkalized alumina capable of adsorbing SO2 by selectively calcining the alumina, and contacting with an alkali or ammonium bicarbonate salt to form at least 30% by weight alkalized alumina having the empirical formula MAl(OH)2CO3.
U.S. Pat. No. 3,865,924 describes the use of a finely ground mixture of potassium carbonate and alumina as an absorbent for carbon dioxide, which reacts with the carbonate and water to form bicarbonate. The absorbent mixture is regenerated by mild heating, e.g. at 93° C. (200° F.). The presence of stoichiometric quantities of water is essential and the alumina appears to be regarded as essentially a mere carrier for the potassium carbonate. Other carbonates may be used.
U.S. Pat. No. 5,232,474 discloses a PSA process using alumina in 70-100% of the bed volume to remove water and carbon dioxide from air. Preference is expressed for alumina containing up to 10 wt. % silica as opposed to the generality of aluminas which typically contain only about 1% silica.
U.S. Pat. No. 5,656,064 discloses treatment of alumina with a base without calcining to form an alkali metal oxide to increase substantially the CO2 adsorption capacity of the alumina that is capable of regeneration under PSA conditions.
U.S. Pat. No. 6,125,655 discloses a TSA process for purifying an air flow containing carbon dioxide and water vapor, in which at least some of the CO2 and water vapor impurities are removed by adsorbing the impurities on at least one calcined alumina containing at most 10% by weight (preferably 4 to 8 wt. %) of at least one alkali or alkaline-earth metal oxide, the adsorption being carried out at a temperature of between −10° C. and 80° C. For PSA cycles, no more than 5 wt. % alkali or alkaline earth metal promotions was preferred.
U.S. Pat. No. 7,759,288 discloses base treated aluminas that exhibit improved CO2 capacity compared to untreated aluminas. The base treated aluminas prepared by physically mixing alumina and base during forming reportedly have (1) a higher surface area, (2) less hydrothermal aging, (3) improved CO2 capacity, and (4) lower cost than base treated aluminas produced by aqueous impregnation.
U.S. Pat. No. 5,656,064 and U.S. Pat. No. 6,125,655 teach that a physical incorporation (incipient wetness impregnation on formed beads or co-formed during bead rolling) of alkali carbonates or oxides on activated alumina enhances CO2 capacity, and is optimized by alkali type, weight loading, and/or surface pH. The method of alkali incorporation can provide differences as well, as U.S. Pat. No. 7,759,288 teaches an improvement in capacity, surface area, and aging stability by co-forming instead of impregnation. The problem with these compositions, in PSA operation, is the loss in capacity between the first and second cycle of CO2 exposure and regeneration purge. As U.S. Pat. No. 5,656,064 shows, CO2 Henry's constants show at best 28% of their original CO2 capacity after just one regeneration under vacuum at ambient temperature.
Activated alumina adsorbents, promoted with alkali (e.g. Na, and K) compounds, are known for removing CO2 from gas mixtures such as air. Alkali compounds increase the basicity of the alumina surface, which increases its affinity for CO2 sorption. It is well-known in the art that CO2 will react with the alkali oxides to form alkali carbonates, and further reaction of CO2 can occur in the presence of water vapor to form alkali bicarbonate.
Industry desires improved adsorbents for removing CO2 from gas mixtures containing low concentrations of CO2.
Industry desires improved adsorbents for capturing CO2 from ambient air.
Industry desires an improved process to produce adsorbents for removing CO2 from gas mixtures.
Industry desires an improved device to produce dry, CO2-free air.
Industry desires an improved device to produce dry, CO2-free synthesis gas.
Industry desires an improved device to produce dry, CO2-free natural gas.